In conventional dispensing of material from a syringe, pressurized air supplied to the upper, larger end of the syringe forces the material out of the lower, smaller end or tip of the syringe. Typically, plastic, disposable syringes used in dispensing of this type are purchased in a liquid-filled condition, with removable plugs connected to both the upper and lower ends of the syringe to contain the liquid.
To dispense liquid from such a syringe, the plugs are removed, a nozzle is placed on the lower end of the syringe, and a compressed air supply is connected to the upper end. A source of compressed air is usually controlled by a timer to supply pressure to the upper end of the syringe in order to dispense liquid out of the nozzle for a selectable time duration.
In many dispensing applications, the amount of liquid dispensed is determined by multiplying an approximate flow rate of discharge by the time duration of dispensing. The flow rate of discharge is calculated from the velocity of discharge flow, the viscosity of the liquid in the syringe and size of the outlet. Basically, because the latter two parameters do not vary substantially during dispensing the contents of a syringe, the flow rate of discharge, in volume per unit time, of the liquid dispensed from the nozzle is dictated by the velocity of discharge flow, in distance per unit time. Thus, the velocity of discharge flow is approximately proportional to the internal pressure of the syringe.
In syringe dispensing of the type described, the internal syringe pressure undergoes an initial ramp increase, remains constant for a while, then undergoes a final ramp decrease to zero. Similarly, as pressure increases, the velocity undergoes a corresponding increase. After the pressure eventually reaches a predetermined, set level, the velocity of discharge flow of the liquid becomes a constant that is proportional to the internal syringe pressure. When the internal pressure of the syringe is decreased, eventually toward vacuum, the velocity gradually decreases. When the internal air pressure reaches zero, dispensing velocity from the nozzle subsequently stops altogether. In other words, the velocity of discharge is not constant throughout an entire dispensing interval, but rather undergoes an initial ramp increase and a subsequent ramp decrease which reflect the corresponding pressure ramps.
This invariably leads to problems in attempting to dispense a precise quantitative amount of liquid from the syringe. If a timer actuated valve is used to supply pressurized air to the top of the syringe from a compressor during a time period of selectable duration, and the time period selected is based upon the assumption that velocity of discharge flow is constant throughout the time period, there will be a discrepancy between the amount of liquid desired to be dispensed and the actual amount dispensed. This discrepancy results directly from the aforementioned ramping of the supplied pressure and the corresponding velocity ramping.
The effects of this problem become more acute when dispensing for very short time periods, where there is an increase in the proportion of the time period in which internal syringe pressure is either ramping up or ramping down with respect to that portion of the time period when the internal syringe pressure is constant.
Stringing, or sticking, of a bead of the liquid to the nozzle also adversely effects the ability to dispense precise quantitative amounts of liquid. Stringing is most likely to occur at lower pressures, for instance when the pressure is ramping up or ramping down. For this reason, stringing also becomes more acute as dispensing time decreases. Stringing of the liquid from the nozzle tip during the final stage of dispensing may be avoided to some extent by making the internal pressure of the syringe negative. However, when dispensing again commences, a build-up of liquid at the nozzle tip almost invariably occurs, thus adversely affecting the stability of the subsequent extrusion.
Additionally, by varying the internal syringe pressure from positive to negative, some evaporation of liquid from the syringe will occur. This evaporation increases the viscosity of the liquid remaining in the syringe. As a result, additional force is required to dispense a desired amount and coverage efficiency is reduced. In short, the increase in viscosity makes it difficult to establish a precise, consistent relationship between applied air pressure and velocity of discharge flow during the course of dispensing the entire contents of a syringe.
Another approach to dispensing fluid from a syringe is disclosed in U.S. Pat. No. 4,784,582, entitled "Fluid Dispensing Pump." This pneumatically operated pump includes an axially movable plunger that extends through the syringe and normally fits into and blocks a dispensing hole. When the plunger is lifted a predetermined amount by an air actuated piston, liquid under pressure flows into the space vacated by the withdrawn plunger. Upon spring actuated return of the plunger to its normal position, a column of liquid in the space is shot through the dispensing hole.
Although this device enables small measured quantities to be dispensed from the syringe, only limited amounts can be dispensed at any one time, with one plunger stroke required for each dispensing amount. Thus, an excessive amount of energy is required in order to dispense the entire contents of a syringe. While the diameter and length of the plunger and plunger column could be increased in order to increase the amount of liquid that can be dispensed in any single plunger stroke, this would also require additional energy to lift the air actuated piston in order to withdraw the plunger.
Another disadvantage results from the fact that, regardless of variation in the viscosities of different liquids that are dispensed from syringes of this type, unless the spring is changed, the down stroke force will always be the same. Thus, the exit velocity, and the liquid coverage will have to be calculated differently if liquids of varying viscosity are used.
It is therefore an object of this invention to improve accuracy, precision and energy efficiency in dispensing quantitative amounts of liquid from a syringe.